Background and motivation: The event of neural Ca2+ overload is known to have several deleterious effects resulting in cell death caused by ischaemia, hypoglycaemia, hypoxia and several neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and AIDS-related dementia. In vitro models for the investigation of the mechanisms involved in Ca2+ overload include brain slice preparations, neuronal cultures as well as acutely isolated neurons, mostly from the hippocampus and cortical brain areas. Additional models for investigating Ca2+ overload may bring about new knowledge to areas of the phenomenon that are still unresolved. Methodology: In this study, several theoretical Ca2+ overload-related interventions were combined aimed at inducing cell death in acutely isolated rat dorsal root ganglia. To elucidate the mechanism/s involved in the cell death observed following exposure to this intervention, the effects of several alterations to the intervention's composition were assessed. This examination was extended by the addition of several recognized and potential protective compounds to the intervention. Cell death was indicated by the trypan blue exclusion assay and recorded after 18 hours exposure to the interventions by counting live and dead neurons under a light microscope. Results and conclusions: The goal was to evaluate the possible application of dorsal root ganglia as a model for neural Ca2+ overload outside the brain. Since Ca2+w as required for cell death to be induced, it is concluded that the observed cell death was indeed primarily due to Ca2+ overload. Besides extracellular Ca2+, KC1-induced depolarization was also required for cell death to be induced, while the antagonists did not demonstrate significant protection against cell death. Based on the results, the mechanism of Ca2+ overload could not be defined beyond doubt, but the voltage activated Ca2+ channels are likely to be involved. / Thesis (M.Sc. (Physiology))--North-West University, Potchefstroom Campus, 2005.

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated ionotropic receptors. When activated, NMDARs let extracellular sodium and calcium ions enter neurons. This calcium influx, depending on its duration, intensity and the presence of nearby signalling proteins can signal to synaptic plasticity. Additionally, physiological NMDAR activity promotes pro-survival cascades and gene transcription, whereas both lack of activation and overactivation of these receptors trigger pro-death signals. Several neurodegenerative pathologies such as stroke/ischemia and Alzheimer’s disease are thought to involve NMDAR overactivation, so-called “excitotoxicity”, but since NMDARs are important for normal neuronal physiology, potential therapeutical approaches needs to go beyond simple antagonism. Here, we studied the receptor subunit composition and the molecular cascades downstream of the receptor activation to try and isolate the pro-death pathways in NMDAR-mediated excitotoxicity. We found that the NR2 subunit composition did not dictate the type of NMDAR-mediated signals, as receptors comprised of NR2B subunits were able to signal to death, survival and plasticity. However, we also found that the intracellular tail of the NR2B subunit was more efficient at triggering neuronal death compared to the NR2A C-terminus, which suggests that different pro-death signalling complexes are associated to each subunit. Two pro-death signals, the p38 and c-Jun N-terminal kinase (JNK) cascades, are key mediators of neuronal excitotoxicity. In a non-neuronal cell line, NMDAR-mediated cell death could be reconstituted but was found to rely solely on JNK and not p38. This was due to the lack of pro-death signals from the NR2B-PDZ domain, a cytoplasmic interacting domain which forms a signalling cassette with the neuronal proteins PSD-95 and neuronal nitric oxide synthase. This PDZ-ligand recruits the p38 cascade in neurons, but was absent in non-neuronal cells. The pro-death p38 pathway could be inhibited in neurons by disrupting the PDZ domain interactions, which protects against excitotoxicity. This disruption was not affecting normal synaptic transmission, potentiation or survival signalling, suggesting that this could be a therapeutically viable avenue. Thus, this work has expanded the understanding of how NMDAR subunits and their cytoplasmic domains mediate signalling leading to a variety of cellular outcomes; a crucial point for the development of a strategy specifically targeting NMDAR- mediated pro-death signalling.

Background and motivation: The event of neural Ca2+ overload is known to have several deleterious effects resulting in cell death caused by ischaemia, hypoglycaemia, hypoxia and several neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and AIDS-related dementia. In vitro models for the investigation of the mechanisms involved in Ca2+ overload include brain slice preparations, neuronal cultures as well as acutely isolated neurons, mostly from the hippocampus and cortical brain areas. Additional models for investigating Ca2+ overload may bring about new knowledge to areas of the phenomenon that are still unresolved. Methodology: In this study, several theoretical Ca2+ overload-related interventions were combined aimed at inducing cell death in acutely isolated rat dorsal root ganglia. To elucidate the mechanism/s involved in the cell death observed following exposure to this intervention, the effects of several alterations to the intervention's composition were assessed. This examination was extended by the addition of several recognized and potential protective compounds to the intervention. Cell death was indicated by the trypan blue exclusion assay and recorded after 18 hours exposure to the interventions by counting live and dead neurons under a light microscope. Results and conclusions: The goal was to evaluate the possible application of dorsal root ganglia as a model for neural Ca2+ overload outside the brain. Since Ca2+w as required for cell death to be induced, it is concluded that the observed cell death was indeed primarily due to Ca2+ overload. Besides extracellular Ca2+, KC1-induced depolarization was also required for cell death to be induced, while the antagonists did not demonstrate significant protection against cell death. Based on the results, the mechanism of Ca2+ overload could not be defined beyond doubt, but the voltage activated Ca2+ channels are likely to be involved. / Thesis (M.Sc. (Physiology))--North-West University, Potchefstroom Campus, 2005.

Myelinated axons are crucial for rapid information transmission within the central nervous system (CNS). Myelin injury is a common feature of white matter (WM) pathology in a number of disease states, including ischemic stroke. Myelin disruption can lead to a complete failure in saltatory action potential conduction, resulting in devastating neurological deficits. However, the fundamental mechanism of ischemic myelin injury is controversial. Glutamate-mediated excitotoxicity is now recognised as a crucial event in the development of ischemic WM pathology. This thesis investigates the potential mechanisms of glutamate release in central WM and examines the hypothesis that NMDA receptor over-activation mediates ischemic myelin damage. Using glutamate biosensor microelectrodes and FM-dye imaging, I show that axonal depolarisation in the adult corpus callosum evokes rapid vesicular docking in axons, capable of elevating extracellular glutamate concentration. My findings show that vesicular fusion occurs under the myelin sheath in myelinated axons, which supports the existence of a novel synapse between the axon and overlaying myelin. Simulation of ischemia triggered an early and robust rise in optic nerve extracellular glutamate levels. Unexpectedly, a significant component of ischemic glutamate release also originated from axonal vesicular fusion. Together, these findings show that the axon-myelin synapse represents a significant site of excitotoxic injury during ischemia. Resolving prior conflicting results, I show that NMDA receptor antagonists prevent myelin degradation and improve functional recovery when applied for sufficient time to penetrate the sheath. Finally, I identify a fluorescent myelin stain (QNZ-46) which is a negative allosteric modulator of NR2C/D-containing NMDA receptors. QNZ-46 selectively accumulates in myelinated WM regions of the CNS following systemic administration, and is retained following wash-out. As a result, QNZ-46 provides persistent protection during ischemia by preserving myelin structure and improving functional recovery.

'Excitotoxicity' represents the excitatory amino acid mediated degeneration of neurons. Glutamate is the major excitatory neurotransmitter in the brain. Glutamate excitotoxicity has been implicated in a number of neurodegenerative disorders like Stroke, Epilepsy, Alzheimer's disease and traumatic brain injury. This neurotoxicity is summed up by the 'glutamate hypothesis' which describes the cause of neuronal cell death as an excessive release of glutamate causing over excitation of the glutamate receptors and subsequent increase in influx of calcium leading to cell death. An effort to counteract this neurotoxicity has lead to the development of glutamate receptor antagonists that can effectively serve as neuroprotective agents. Nevertheless, the downside to these drugs has been the side effects observed in clinical trial patients due to their disruptive action on the physiological function of these receptors like learning and memory. This work was undertaken to identify targets that can effectively be used to treat excitotoxicity without affecting any normal physiological functions. In one approach, (chapter I) we have identified the KATP channels as an effective modulator of epileptogenesis. In another approach, (Chapter II) we show that targeting the AMPA receptor subunit GluR2 is a practical strategy for stroke therapy. KATP channels that are gated by intracellular ATP/ADP concentrations are a unique subtype of potassium channels and play an essential role in coupling intracellular metabolic events to electrical activity. Opening of KATP channels during energy deficits in the central nervous system (CNS) induces efflux of potassium ions and in turn hyperpolarizes neurons. Thus, activation of KATP channels is thought to be able to counteract excitatory insults and protect against neuronal death. Here, we show that, functional Kir6.1 channels are located at excitatory pre-synaptic terminals as a complex with type-1 Sulfonylurea receptors (SUR1) in the hippocampus. The mutant mice with deficiencies in expressing the Kir6.1 or the SUR1 gene are more vulnerable to generation of epileptic form of seizures, compared to wild-type controls. Whole-cell patch clamp recordings demonstrate that genetic deletion of the Kir6.1/SUR1 channels enhances glutamate release at CA3 synapses. Hence, expression of functional Kir6.1/SUR1 channels inhibits seizure responses and possibly acts via limiting excitatory glutamate release. In addition to epilepsy, ischemic stroke is a leading cause of death in developed countries. A critical feature of this disease is a highly selective pattern of neuronal loss; certain identifiable subsets of neurons, particularly CA1 pyramidal neurons in the hippocampus are severely damaged, whereas others remain intact. A key step in this selective neuronal injury is Ca2+/Zn2+ entry into vulnerable neurons through [alpha]-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor channels, a principle subtype of glutamate receptors. AMPA receptor channels are assembled from glutamate receptor (GluR) -1, -2, -3, and -4 subunits. Circumstance data have indicated that the GluR2 subunits dictate Ca2+/Zn2+ permeability of AMPA receptor channels and gate injurious Ca2+/Zn2+ signals in vulnerable neurons. Here we show that ischemic insults induce toxic Ca2+ entry through AMPA receptors into vulnerable neurons by modification of GluR2 RNA editing. Thus, targeting of GluR2 subunit can be considered as a promising target for stroke therapy.

<div>Glutamate is the principal excitatory neurotransmitter in the central nervous system. As one of the most abundant neurotransmitters, glutamate plays an essential role in many processes of the central nervous system and beyond. As a result, any disruption that causes an abnormal glutamate level can significantly impact the central nervous system's neurological functions. Glutamate excitotoxicity is a neuropathology that persists in many neurodegenerative disorders such as Parkinson's and Alzheimer's disease as well as in the traumatic brain and spinal cord injuries. Thus, the ability to obtain precise information about the extracellular glutamate level in the living brain and spinal cord tissue may provide new insights into the fundamental understanding of glutamate in neurological disorders and neurophysiological phenomena.</div><div><br></div><div>Conventional bioanalytical techniques that characterize glutamate levels <i>in vivo</i> have a low spatiotemporal resolution that has impeded our understanding of this dynamic event. The electrochemical sensor has emerged as a promising solution that can satisfy the requirement for highly reliable and continuous monitoring methods with an excellent spatiotemporal resolution for the characterization of extracellular glutamate concentration. In this thesis, I present various amperometric biosensors fabricated using a simple direct ink writing technique for<i> ex vivo </i>and <i>in vivo</i> glutamate monitoring.</div><div><br></div><div>The amperometric biosensor is fabricated by immobilizing glutamate oxidase on nanocomposite electrodes made of platinum nanoparticles, multiwalled carbon nanotubes, and a conductive polymer. The biosensors demonstrate good sensitivity and selectivity that can be inserted into a spinal cord and measure extracellular glutamate concentration. Additionally, another type of glutamate biosensor is fabricated from commercially available activated carbon with platinum microparticles. We utilize astrocyte cell culture to demonstrate our biosensor's ability to monitor the glutamate uptake process. We also present a direct measurement of glutamate release from optogenetic stimulation in mouse primary visual cortex brain slides. </div><div><br></div><div>Moreover, we explore a new type of material, perovskite nickelate-Nafion heterostructure, to fabricate biosensors and measure glutamate inside the mouse brain. Finally, by utilizing the nanocomposite ink and direct ink writing technique, we also fabricate the gold-ruthenium non-enzymatic glucose biosensor. We apply a modified Butler-Volmer non-linear model to evaluate the impact of geometrical and chemical design parameters of non-enzymatic biosensor performance. </div><div><br></div>

Glutamate carboxypeptidase II (GCPII), also known as prostate-specific membrane antigen (PSMA), is a membrane metallopeptidase overexpressed on most prostate cancer cells. Additionally, GCPII also attracted neurologists' attention because it cleaves neurotransmitter N-acetyl-L-aspartyl-L-glutamate (NAAG). Since NAAG exhibits neuroprotective effects, GCPII may participate in a number of brain disorders, which were shown to be ameliorated by GCPII selective inhibitors. Therefore, GCPII has become a promising target for imaging and prostate cancer targeted therapy as well as therapy of neuronal disorders. Globally, prostate cancer represents the second most prevalent cancer in men. With the age, most men will develop prostate cancer. However, prostate tumors are life threatening only if they escape from the prostate itself and start to spread to other tissues. Therefore, considerable efforts have been made to discover tumors earlier at more curable stages as well as to target aggressive metastatic cancers that have already invaded other tissues and become resistant to the standard treatment. Since patients undergoing a conventional therapy (a combination of chemotherapy and surgery) suffer from severe side effects, more effective ways of treatment are being searched for. Novel approaches include selective...